Phosphonic acids, defined by the presence of stable carbon-phosphorus (C-P) bonds, are an underexploited group of compounds with biomedical promise. Some the most interesting and useful members of this class are natural products, including ones with potent antibacterial, antifungal, and antiviral activities. The great biomedi- cal potential of phosphonic and phosphinic acids, stems from (1) a proven track record in clinical and commer- cial applications, (2) an immense diversity of cellular targets and modes of action (3) availability of a suite of research tools that allow facile discovery and characterization, (4) a proven reservoir of novel, uncharacterized members of the class, and (5) unusual biosynthetic pathways whose characterization has enhanced and ex- panded our knowledge of fundamental biochemistry. This research project takes advantage of these features, focusing on phosphonate natural products from actinobacteria with useful bioactivities and interesting chemical features, as well as the discovery of novel phosphonates. The long-term goals of the project include elucidation of the genes, biosynthetic pathways, resistance mechanisms and bioactivities of selected phosphonates and development of tools for efficient characterization and production of bioactive C-P compounds. The specific aims for the proposed funding period are: (1) characterization of the biosynthesis of two hybrid polyke- tide/phosphonate natural products, (2) characterization of the biosynthesis of a rare H-phosphinate, (3) charac- terization of the biosynthesis of an unusual branched chain phosphonate and (4) development an optimized heterologous expression host to aid the discovery and production of novel phosphonic acid natural products.